1. Field of the Invention
The present invention relates to a monitor apparatus for monitoring activity of the respiratory muscles of a patient whose breathing is partially supported by a lung ventilator which interacts with patient's respiratory muscles.
2. Description of the related Art
In a case where a lung ventilator is used, particularly in pressure support ventilation (PSV) mode, on a patient who manages to breathe spontaneously, the PS level and the amount of air supplied by the lung ventilator must be adjusted suitably to the condition of the patient so as to promote the spontaneous breathing and, at the same time, substantially prevent fatigue of the patient's respiratory muscles.
In usual practice, the operational conditions of a lung ventilator are determined according to the physical conditions of the patient, such as blood gases, the respiration rate and the volume of ventilation, which are being monitored. A paper "Mathematical Model of Pressure Support Ventilation--an Explication of Mechanical Interaction between a Lung Ventilator and Respiratory Muscles," presented by the present inventors in the 38th Annual Meeting of Japan Society of Anesthesiology, Mar. 28, 1991, suggests that the work of respiratory muscles can be obtained by assuming an equivalent circuit of the respiratory system connected with a lung ventilator, as shown in FIG. 7. In the figure, E is a pressure of the supplied air, e is a spontaneous breathing source of a lung, Paw is an air pressure at the airway opening, Rl is an air flow resistance of the lung, and El is an elastance (an inverse of the compliance C) of the lung. A pressure inside a thoracic cavity Ppl is measured with assistance of a balloon catheter inserted in the patient's esophagus. An air-flow rate dV/dt is detected by a flow rate sensor which is installed in an air passage of the lung ventilator connected to the respiratory system. Patient's work W is calculated by integrating the value calculated by multiplication of the pressure and flow rate, as below EQU W=.intg.-PpldV/dtdt
However, such a method of obtaining the work has several problems. The method is invasive. Measurement precision is not satisfactory since the work performed by the respiratory muscles to drive the thorax is not considered in the method. Further, supplying air of the lung ventilation may sometimes be late for the initiation of inspiration performed by the respiratory muscles, in which case the work performed by the respiratory muscles is wasted. There has not been developed a method to monitor the timing of the inspiratory activity of the respiratory muscles in order to initiate air-supplying of the lung ventilator exactly when PS level is developed.